Apparatus for separating mixtures having particles of various sizes are known. For example, U.S. Pat. No. 433,096 showed a flour separator. The device included two counter-rotating, concentric, cylindrical screen drums. The drums were downwardly inclined from the input to the output ends with respect to a horizontal line to utilize the gravitational force to keep flour flowing. Flour having particles of various sizes was introduced into the inner drum at the input end. The inner drum had screen with openings which prevented coarse particles of flour from passing therethrough. The outer drum had screen with openings which allowed only small or fine particles to pass therethrough. As the flour proceeded, the fine particles dropped through both screen drums into a trough having an auger therein which moved the fine particles to a spout. The middling particles dropped through the screen openings of the inner screen drum only and proceeded to the output end of the outer drum to fall into a spout. With the output end of the inner drum extending beyond the output end of the outer drum, the coarse particles proceeded to the output end of the inner drum and dropped into a spout. An expensive-appearing planetary gear arrangement was used to drive the screen drums in counter-rotating directions.
Grain cleaning separators are also known. The known devices, however, have included concentric screen drums rotating in the same direction at the same speed. As a result, the devices have had limited capacity. When both drums operated at the same rotational speed, the surface speed of the inner drum was slower than the surface speed of the outer drum. As a consequence, grain was processed more slowly through the inner drum than the outer drum, restricting the rate of flow to the capacity of the slower, inner drum. This problem has been addressed and solved in the present invention.